I don't think anyone is going to be able to suggest a bike light that is "not stealable".

I just got a Lumotec Cyo installed on my bike, it runs off my alfine equipped front hub. It's noticeable mounted on the handlebars, but I'm not sure how it wouldn't be. If it's mounted on the fork I imagine it would be a lot less noticeable. Has an on/off switch.